How do you balance possibly doing a season 5 of this versus wanting to do other things?
Everyone involved in the show is acutely aware of how well-written it is and how well-produced it is. If you are on a show that is well-written, well-shot, well-scored, and you’re lucky enough to have chemistry with the person, that’s pretty cool. And that doesn’t come along every day. But at the same time, we do want to do other stuff. And thank God we’re all busy. [Co-creators] Steven Moffat and Mark Gatiss are busy. Everyone’s busy. I wouldn’t want to make any promises either way on that.

I think things ending is not a thing to be afraid of. Can you imagine the Beatles now? Jesus. Certain things should end. I think everything is supposed to be finite. Who knows? I never really know once we’ve finished a series, because it takes a long time to make another one.